The relative corneal toxicity of antiglaucomatous drugs used routinely in ophthalmology was assessed in a microculture technique. DNA and protein synthesis of monkey corneal endothelial cell cultures were markedly inhibited by the addition of 25 microgram/ml epinephrine, 0.5 mg/ml timolol maleate, or 2.5 mg/ml pilocarpine. Stromal and epithelial cultures were not affected by a concentration of 2.5 mg/ml pilocarpine or 50 microgram/ml epinephrine. The metabolism of stromal and epithelial cells was totally inhibited by a concentration of 2.5 mg/ml timolol maleate. Scanning electron microscopic studies of the above cultures revealed a marked distortion of the cellular structures at the inhibiting concentrations.